Cell surface carbohydrates change during development as they are probably involved in differentiation. Thesse developmentally-regulated changes allow some antibodies directed against carbohydrates to discriminate among tissues, both normal and malignant. To obtain monoclonal antibodies specific for various cell types, mice and rats have been immunized with human tissues in many laboratories. Some of the antibodies derived from spleen cells of the immunized animals animals have an apparent specificity for certain cancers. Out of 460 of these antibodies sent to us from several laboratories in the past 4 years, 121 are directed against carbohydrate sequences in glycoproteins and glycolipids as determined by solid-phase radioimmunoassay and by immunostaining of thin layer chromatograms of glycolipids. Of the 121 antibodies, 23 are directed against human blood group antigens including 7 against the H type 1 or H type 2 antigens, 4 against the Le-b antigen, 1 against the Le-a antigen, 8 against the A antigens, and 3 against the B antigen. Fifty-six antibodies are directed against a sugar sequence found in the human milk oligosaccharide lacto-N-focopentaose III, 1 against a sugar sequence found in the human milk oligosaccharide lacto-N-neotetraose, 2 against a sialylated Le-a antigen, and 40 against unidentified carbohydrate sequences in glycolipids and/or glycoproteins.